(1) Field of the Invention
The present invention relates to a process for the preparation of aromatic hydroperoxides in which an aromatic compound having a secondary alkyl group is subjected to liquid phase catalytic oxidation with a molecular oxygen-containing gas in the presence of a copper compound catalyst and a basic aqueous solution, whereby the rate of oxidation of the aromatic compound is increased and a corresponding aromatic hydroperoxide is prepared in a high yield.
(2) Description of the Prior Art
A process for preparing aromatic hydroperoxides by subjecting an aromatic compound having a secondary alkyl group, such as cumene, cymene, dimethylcumene, diisopropylbenzene or sec-butylbenzene, to liquid phase oxidation with a molecular oxygen-containing gas in the presence of a basic aqueous solution is well known, and this process is industrially carried out for the preparation of precursors of phenols. Also a process for preparing aromatic hydroperoxides by subjecting an aromatic compound having a secondary alkyl group, such as mentioned above, to liquid phase oxidation with a molecular oxygen-containing gas in the absence of a basic aqueous solution is known. Since the rate of this liquid phase oxidation is low in these known processes, there have been proposed processes for increasing the oxidation rate in which the liquid phase oxidation reaction is carried out in the presence of a catalyst of a compound of a heavy metal such as copper, manganese, cobalt, nickel or iron (see the specifications of British Pat. No. 665,897, British Pat. No. 665,898, British Pat. No. 745,128, British Pat. No. 760,367, British Pat. No. 801,387 and U.S. Pat. No. 2,954,405, Japanese Patent Publications No. 36899/70, No. 38604/70, No. 6568/71 and No. 7148/74, and Japanese Patent Application Laid-Open Specifications No. 507/73 and No. 72225/74). However, in each of these processes, the rate of the liquid phase catalytic oxidation is not sufficiently high. Especially when the aromatic compound to be oxidized is an aromatic compound having a primary alkyl group as well as a secondary alkyl group, such as cymene, dimethylcumene or diisopropyltoluene, the oxidation rate is much lower than in case of an aromatic compound having a secondary alkyl group alone, and furthermore, a primary aromatic hydroperoxide or secondary aromatic hydroperoxide is competitively formed as the oxidation product in addition to an intended tertiary aromatic hyroperoxide, resulting in reduction of the selectivity of the oxidation reaction. As means for improving the selectivity in the liquid phase catalytic oxidation of an aromatic compound having a secondary alkyl group and a primary alkyl group, there has been proposed a process in which the liquid phase catalytic oxidation is carried out in the presence of a heavy metal compound catalyst and an aqueous phase by using a molecular oxygen-containing gas (see Japanese Patent Application Laid-Open Specifications No. 142526/75 and No. 142527/75). According to these proposals, especially the proposal of Japanese Patent Application Laid-Open Specification No. 142,527/75, a chelate compound of a heavy metal is used as the catalyst and the liquid phase catalytic reaction is carried out while adjusting the pH value of the aqueous phase in the weakly acidic or neutral region, that is, to 5 to 7.5. According to this process, the selectivity to the tertiary aromatic hydroperoxide is improved to some extent, but decomposition of the aromatic hydroperoxide is advanced considerably in the catalytic oxidation system and phenol compounds are formed as by-products. Since these phenol compounds rather inhibit the oxidation reaction, there is brought about a problem of reduction of the oxidation rate.